The new woody matter being deposited outside the old, between the bark and the previously formed layer, gives origin to the term exogen (ἔξω, outside, γεννἁω, to produce). These structures may be examined in the section of a branch of the lime-tree or lilac. In the _Monocotyledons_ or _Endogens_ (Pl. I. fig. 37), there is no distinct bark, nor pith, nor medullary rays--the entire stem consisting of cellular tissue with isolated bundles of fibro-vascular tissue scattered through it. Moreover the new substance is added to the centre of the stem, or within the old; hence the term endogen (ἔνδον, within, γεννἁω). A section of a piece of cane will exhibit this structure. To examine the structure of stems, sections must be made in various directions. The relative position of the component parts of a stem are best seen in a transverse section; but the structure of the tissues is most evident in longitudinal sections, and under the higher powers. The annual rings of the Exogens are best observed in transversely sawn-off pieces of perfectly dry stems, which have been polished with sandpaper, and varnished with spirit varnish. _Roots._--The structure of roots is very similar to that of stems; there is, however, no distinct pith, nor are there stomata on the epidermis; and the vessels are replaced by ducts. The very fine rootlets or radicles of water-plants often show the rotation of the protoplasm very distinctly. _Flowers._--The various parts of flowers, being each a modified leaf, present the same general structure as the latter. As the reader may not be acquainted with the names of these parts or organs in the higher plants, and as we shall have to compare them with their representatives in the lower forms of vegetable life, it will be well briefly to indicate them. A common and beautiful yet despised flower (Pl. I. fig. 32) may serve for illustration; this is chickweed (_Stellária média_), which can be found everywhere. The outermost circle of flower-leaves, which forms a kind of cup to the rest of the flower (_a_), is the _calyx_; the separate leaves being called the _sepals_. The row within this, in most flowers consisting of brilliantly coloured pieces, forms the _corolla_ (_b_); the individual pieces being the _petals_. When the two kinds are equally coloured, or not distinguishable, the whole is called the perianth, as in a tulip. When the segments of the _perianth_ are dry and chaffy, as in the flowers of grasses, the outermost are said to constitute the _glumes_, and the innermost the _paleæ_. Within the ring of petals are certain thread-like organs called _stamens_ (_c_); and these consist of a _filament_ (fig. 39 _a_), surmounted at the top or apex by the _anther_ (fig. 39 _b_), which is usually coloured, and consists of two lobes. The anthers when ripe burst, and discharge a coloured dust; this is the _pollen_. Lastly, within the stamens is the central organ of the flower, the _pistil_, and sometimes there are several of them.